The present invention relates to targets useful in the fabrication of thin films of quasicrystalline materials.
Quasicrystalline materials constitute a class of alloy materials that possess five-fold or ten-fold symmetry. Earlier research indicates that these materials may exhibit properties that could have many commercial and scientific benefits.
Quasicrystalline materials have been fabricated in bulk form as ingots with dimensions on the order of less than a centimeter on a side. Two of the alloy stoichiometries that have been found to exhibit quasicrystalline properties are Al71.1Cu5Fe11.3Cr12 and Al65Cu23Fe12. Numerous other alloy compositions have also been found to exhibit quasicrystalline properties.
Quasicrystalline materials have also been produced as thin films by thermal spray deposition. Films produced in this manner characteristically contain porosity and exhibit a rough surface topography. It is desirable to produce high density, low porosity thin films of these materials for optimal performance. The thin film deposition models suggested herein are presented to aid visualization of the thin film deposition process and are not intended to limit in any way the useful range of the disclosed invention.
In the implementation of any thin film deposition process, energy is applied to a quasicrystalline material or to separate materials comprising the individual atomic constituents of the desired quasicrystalline stoichiometry. This applied energy vaporizes or sputters the target materials. The vaporized or sputtered constituents fill the environment, usually a vacuum vessel, in which the target materials and the substrate to be coated are located. Some fraction of the vaporized or sputtered material which comes into contact with the substrate adheres thereto. In this manner, a thin film of quasicrystalline material develops on the substrate over a period of time as it is exposed to the vaporized or sputtered material(s).
Two methods have generally been used in the deposition of high density, low porosity thin films of quasicrystalline materials. According to the first method a multiplicity of targets are used, each target comprising one constituent of the quasicrystalline alloy being deposited. For example, in order to deposit thin films of the alloy composition Al71.1Cu5Fe11.3Cr12, four distinct targets are used: one entirely aluminum Al, one of copper Cu, one of iron Fe and one of chromium Cr. The substrate is passed in front of each target sequentially for a brief period of time during which the target is ablated by appropriate heavy ion bombardment. Each ablated atomic species then impacts the substrate to which it adheres. The process is repeated until the desired total thin film thickness is achieved.
The difficulty with this method is that the resulting film can evolve into a quasicrystalline film only by the interdiffusion of the individual atoms. Thus, in each pass in front of the set of targets, atoms of the first deposited species must diffuse through the layer deposited by the last deposited species, and atoms of the last deposited species must diffuse through the layer produced by the first deposited species. Thus, the alloy composition and uniformity of the film depends entirely on the success of this interdiffusion process. This process has only been successfully used for very thin quasicrystalline films on the order of 300 nanometers or less because of interdiffusion issues with layers and substrate elements.
In the second method, the source of ablated or vaporized material is a solid ingot or plate of quasicrystalline material possessing the stoichiometry of the desired thin film. Energy may be applied from a thermal source, laser source or by impact from ions of an inert material such as argon or xenon. Laser energy sources have not been used extensively. Impact by ablating ions can be achieved by means of conventional ion sources or by magnetron sputtering. In the latter method, argon ions or other heavy ions such as xenon are accelerated to high energies and impact the quasicrystalline target thereby transferring both energy and momentum from the ions to the target material in accordance with the conservation laws of physics. The momentum and energy acquired by the quasicrystalline target material is chosen to be sufficient to free individual alloy atoms or particles from the quasicrystalline target. These atoms or particles then traverse the intervening space between the target and the substrate. Upon impact with the substrate, momentum and energy is exchanged with the substrate. In this manner, a film of the quasicrystalline material develops on the substrate over a period of time.
It has been observed, however, that energy transferred to the quasicrystalline target from the incident ions results in local heating of the target. Unfortunately, bulk quasicrystalline materials such as target ingots are extremely brittle. Therefore, in all experiments performed with this technique to date, this localized heating has produced a thermal shock that causes the target to fracture making it unusable for further thin film deposition. This low thermal shock resistance of quasicrystalline materials is an inherent property of all such alloy compositions.
Thus, the prior art has yet to present a satisfactory method for the deposition of thin films of quasicrystalline alloy materials directly from quasicrystalline alloy materials.
It is therefore an object of the present invention to provide a method for the deposition of quasicrystalline films directly from improved and novel target materials.
It is another object of the present invention to provide improved targets for the formation of quasicrystalline films that do not exhibit the thermal shock sensitivity of prior art such targets.
It is yet a further object of the present invention to provide a method for the fabrication of targets for the production of quasicrystalline thin films.
According to the present invention, targets for the fabrication of quasicrystalline films are prepared from powders of the elemental constituents of the objective quasicrystalline material that have been pressed into a required target shape. The temperature of target fabrication is maintained sufficiently low that quasicrystalline alloy formation does not occur. Due to the high thermal shock resistance of each of the individual constituents and due to the dispersed form of the powders comprising the target, the target demonstrates very high resistance to thermal shock.